Tunable Liquid Crystal (TLC) optical devices are described, for example, in related International Patent Application WO/2007/098602, which claims priority from U.S. Provisional Patent Application Ser. No. 60/778,380, filed Mar. 3, 2006, both of which are incorporated herein by reference. TLC optical devices are typically flat multi-layered structures having at least one Liquid Crystal (LC) layer. The liquid crystal layer has a variable refractive index which changes in response to an electromagnetic field applied thereto. In general, TLC's are said to have an index of refraction which varies as a function of an applied drive signal producing the electromagnetic field. Applying a non-uniform (spatially modulated) electromagnetic field to such liquid crystal layer, provides a liquid crystal layer with a non-uniform (spatially modulated) index of refraction. Moreover, liquid crystal refractive index variability is responsive to a time variable electric field and the liquid crystal layer exhibits negligible to non-measurable hysteresis with respect to achievable optical parameters given sufficient time. This is a significant advantage over many other autofocus systems, including most mechanical systems, because the TLC will attain the same stable optical parameter level no matter from which direction that level is approached (i.e. no matter from what optical parameter level the TLC starts to change, whether it be lower or greater), and regardless of any previous optical parameter changes. This means that once an optical parameter level is passed, no excessive amount of “backtracking” in terms of control is necessary (e.g. given sufficient time, there is no need for “reset” so that optical parameter changes can proceed to a targeted level).
The nature of the variability of the index of refraction in response to an applied electromagnetic field depends on the physical properties of TLC multi-layered structure, including properties of the liquid crystal layer material, geometry, etc. A quasi-linear “functional” relationship between the drive signal applied and the index of refraction of a TLC optical device exists over a usable drive signal variability range. However, the overall relationship is non-linear: In some TLC devices, an abrupt change in liquid crystal orientation, known as disclination, is observed as the liquid crystal molecules begin to align with the electric field from a ground state orientation to an orientation dictated by the applied field. In broad terms, when the applied field is essentially homogenous, non-linearity means that the change in optical property (e.g. index of refraction) per unit drive signal change varies over the range of optical property change of the optical device.
A multitude of optical devices may include a TLC optical device. For example, with an appropriate geometry a tunable lens, a beam steering device, an optical shutter, etc. may be built. A Tunable Liquid Crystal Lens (TLCL) provides a lensing effect by creating regions of differing indices of refraction in the liquid crystal layer when subjected to an electromagnetic stimulus, for example creating a Gradient Index Lens.
Tunable lenses employing a TLC optical device offer the advantage of being thin and compact. Factors such as thickness and size are important in certain applications, such as in the case of handheld equipment including, but not limited to: mobile telephone cameras, inspection equipment, etc. The performance of TLC lenses may be measured by a multitude of parameters, including: a tunable focus range, optical power (diopter) range, a level of aberration, an auto-focusing speed, power consumption, etc.
Different approaches have been proposed for providing tunable liquid crystal lenses, for example:
A notable prior art experimental attempt at providing a TLC lens is Naumov et al., “Liquid-Crystal Adaptive Lenses With Modal Control” Optics Letters, Vol. 23, No. 13, p. 992, Jul. 1, 1998, which describes a one hole-patterned layered structure defined by a non-conductive center area of an electrode covered by a transparent high resistivity layer. With reference to FIG. 1A, TLC 100 includes: top 102 and bottom 104 substrates, and a middle Liquid Crystal (LC) layer 110 sandwiched between top 112 and bottom 114 liquid crystal orienting layers. LC orienting layers 112/114 include polyimide coatings rubbed in a predetermined direction to align LC molecules in a ground state, namely in the absence of any controlling electric field. The predetermined orientation angle of LC molecules in the ground state is referred to herein as the pre-tilt angle. The average orientation of long liquid crystal molecular axes in a liquid crystal layer is referred to as a director. An electric field is applied to the LC layer 110 using a uniform bottom transparent conductive electrode layer 124 of Indium Tin Oxide (ITO), and the top hole-patterned conductive ring electrode layer 122 of Aluminum (Al). The low resistivity hole-patterned conductive layer 122 together with the high resistivity layer 126 immediately below the hole-patterned conductive layer 122 form an electric field shaping control layer 128. In accordance with Naumov's approach, the reactive impedance of the LC layer 110 which has capacitance and the complex impedance of the high resistivity layer 126 play a strong role, requiring driving the TLCL via specific voltage and frequency parameter pairs to minimize root means square deviation from a parabolic phase retardation profile for corresponding desired optical power settings (transfer function) to gradually spatially shape (spatially modulate) the applied electrical field otherwise spanning between the uniform bottom transparent electrode 124 and the hole-patterned top electrode 122.
Unfortunately, from a manufacturing perspective it is very difficult to re-produce the required sheet resistance of high resistivity material with high optical transparency for the highly resistive layer 126, and therefore in practice it is very difficult to re-produce a TLCL in accordance with the geometry described by Naumov. The manufacturing process typically suffers from a very low yield. Different TLCL's of the same manufacturing batch have slightly different resistances. Such sheet resistance variability coupled with the fact that control is very dependent on the precise LC cell thickness, leads to each individual TLC lens requiring separate calibration and drive. Also, the minimum diameter of such a TLC lens is limited to about 2 mm—below this size the required resistivity of the ITO layer exceeds some 10 MΩ/sq.
Another notable prior art experimental attempt at providing a TLC lens is Sato et al., “Realization of Liquid Crystal Lens of Large Aperture and Low Driving Voltages Using Thin Layer of Weakly Conductive Material”, Optics Express, Vol. 16, No. 6, p. 4302, 17 Mar. 2008, which describes a layered structure 200 having three flat electrodes in two groups, as shown in FIG. 1B. Two patterned electrodes form one group, and a single uniform electrode forms the other group. Compared to Naumov, Sato describes an additional transparent disc-shaped electrode used to provide relatively uniform electrical fields across the LC layer 110 when needed and a weakly conductive layer (WCL). Electric field shaping control layer 228 differs from that of Naumov in that the top substrate 202 and the top electrode 222/230 (group) are present in reverse order. The top electrode group includes distinct electrodes 222 and 230 in an inter-hole pattern formed in the same plane. Electrode 222 is a hole-patterned ring electrode of conductive Al, while the center electrode 230 in the top group is a fixed disk-shaped transparent conductive layer of ITO. Two drive signals U_ring and U_disk are employed. The role of the hole-patterned electrode 222 with voltage U_ring applied thereto is to create a lensing electric field profile, while the role of the central disk-shaped electrode 230 with voltage U_disk applied thereto is to reduce disclinations and to control the electric field gradient (e.g., to erase the lens). The WCL 226 in this configuration allows close positioning of the top (patterned) electrode to the bottom ITO electrode 124, thus reducing required voltages.
Unfortunately, the complex patterning of the top electrode, the necessity of using two distinct drive signal voltages and a separate WCL 226 are difficult to manufacture as a unit and inhibit practical use of this approach. For example, the use of this approach to build a polarization independent lens would require the use of six to seven thick glass lens elements.
Both of the above mentioned approaches suffer from additional drawbacks. In using Naumov's approach, the performance of such a TLC lens is very sensitive to the thickness of the LC cell as well very sensitive to the sheet resistance R_s of the highly resistive layer 126. It happens that, for millimeter size lenses, the value of R_s, for almost all known solid state materials, is in the middle of an electrical conductivity transition (percolation) zone, where the sheet resistance has a very drastic natural variation with layer 126 geometry (thickness). Thus, it is extremely difficult to achieve consistency (repeatability) in building highly resistive layers 126 with the same R_s.
Each of Naumov's and Sato's approaches require the use of two highly resistive layers 126 or WCLs 226 to build polarization independent lenses. Thus, the problems of R_s reproducibility and complexity drastically reduce manufacturing yields and increase manufacturing costs.
As mentioned, prior art tunable LC lenses employ a driving signal having an adjustable voltage to change the optical properties of the LC layer. As mentioned above, another problem with prior art systems having patterned electrodes is the effect of “disclination.” When using a spatially non-uniform voltage for tuning a TLC lens the initial voltage increase creates non-uniform electric field lines that cause some of the LC molecules to realign differently than others which experience the same electric field strength. Such disclinations cause optical aberrations in the lens which persist with gradual voltage adjustments necessarily employed in tuning. Such disclinations can be removed (in Sato's approach) by aligning all molecules with a very high voltage pulse that erases the lens, before reducing the voltage back to the appropriate range for providing a desired optical power, however such high voltage pulses are undesirable for example due to operational parameter violations of the overall device.
Auto-Focus (AF) is a process implemented in many camera systems to enable easier focus acquisition for camera users, sparing them of the need to manually focus a scene. Handheld digital camera operation in auto-focus mode is negatively affected by both increased power consumption and slow response speed, factors which further negatively influence each other. An important performance characteristic of auto-focus operation is the maximum time taken by the focus acquisition process to complete. Auto-focus applications, such as handheld camera systems require good auto-focus speed performance.
Auto-focus systems are used with TLC lenses where the optical power of the TLC lens is changed by applying a drive signal to the TLC lens as indicated by an auto-focus algorithm. In contrast with conventional focusing systems, TLC lenses remain stationary at all times. For image focusing purposes, an optical power of a TLC lens refers to the amount of bending (convergence) that the TLC lens imparts to incident light (and more specifically to an incident light field referred to as a scene) passing therethrough.
There are a number of algorithmic techniques which can be employed to compute convergence to an optical power setting corresponding with best focus scores for a given scene. Auto-focus algorithms implement a so called full search approach, hill climb approach, etc. Auto-focus speed is in part dependent on the optical power change speed.
The full search algorithm typically involves adjustment of the tunable lens across its full range of optical power in small and even drive signal adjustment steps. Focus scores are determined and recorded for each step, the variation of focus scores with either drive signal level or optical power is referred to as a focus curve. A maximum of the focus score variation (curve) is determined, and the optical power of the (TLC) lens is set to correspond to that for the maximum focus score. This technique is also referred to as staircase, because the up and down drive signal adjustment steps employed resemble a staircase. One drawback to this algorithm is that in practice implementations are slow. Each small step requires a non-trivial amount of time to complete, and the aggregate number of steps can take up a substantial amount of time. Moreover, the required traversal of the entire optical power range and therefore the traversal of the entire drive signal control range to implement the full search algorithm for a variable voltage controlled TLC lens may leave the molecules of the LC layer in a saturated high power state at the end of the focus search. Employing the full search algorithm with a voltage controlled TLC lens typically further suffers from a slow response time due to slow LC molecular relaxation from the required LC molecular saturated state of the highest voltage applied at the end of the full search to a lower voltage moderate power state needed to subsequently acquire the image at best focus. The slow response time is not only undesirable but variable. The more the maximum focus is found at extreme drive signal voltages, the longer the relaxation time required.
The hill climbing algorithm employs a technique for detecting which optical power setting corresponds to a peak focus score. This technique assumes that there will be a single peak in a focus score curve varying with optical power. This is typically considered a safe assumption in naturally occurring scenes in consumer photography and video. The general shape of such a focus score curve resembles a hill. With reference to FIGS. 2A and 2B, the hill climbing algorithm involves stepping through at least a portion of the optical power range of the tunable lens while detecting the climb up the hill in terms of focus scores, and then, immediately after the peak is passed (indicated by a drop in the focus scores), pulling back to the optical power level observed at the focus score peak. One approach to the hill climb technique involves selecting substantially equally spaced samples across the adjustable optical range, as illustrated in FIG. 2A.
Because the hill climbing algorithm aborts the focus search after the focus score peak is detected, the overall number of steps can be reduced thereby reducing the focus acquisition time delay. Because the hill climb approach essentially stops and retreats a bit after it passes over the peak, the amount of time it takes to complete in this procedure depends on how far into the optical range the peak is located. If the focus peak is near the beginning of the optical range sweep, the procedure involves a relatively few steps. However, if the focus peak is near the end of the optical range sweep, the procedure involves relatively more steps. To reduce the amount of time the entire procedure takes on average, it may be beneficial to make larger steps at the beginning of the focus scan sweep than those later in the focus scan sweep, as illustrated in FIG. 2B. Far few steps are taken to reach peaks positioned further away from the beginning of the optical range sweep, thus taking less time to complete the entire procedure. In those cases where the best focus score peak is nearer the beginning of the optical range, additional time may be taken to perform larger backward steps, but this is acceptable because the procedure will have spent relatively little time traveling to that early backtrack position. Compared, to a full scan algorithm, the hill climbing algorithm only involves sweeping across the entire optical range for scenes requiring a focal distance outside the focus range of the optical system (typically too close).